The present invention relates to a method of obtaining metallic iron by subjecting iron oxides contained in iron ore or the like to reduction through the application of heat using a carbonaceous material as a reductant. More specifically, the invention relates to a method of efficiently making high purity metallic iron in which iron oxides are efficiently reduced into metallic iron while slag components including gangue and the like contained in an iron oxide source, such as iron ore, are melted and separated properly from metallic iron, and to a method and apparatus for industrially making metallic iron based on this method.
A conventional method of making direct reduced iron is where iron ore or pellets which contain iron oxide are directly reduced using a reducing gas to obtain reduced iron. An example is a shaft furnace method represented by the Midrex process. In this type of method of making direct reduced iron, a reducing gas made from natural gas or the like, is forced into a shaft furnace from a tuyere located at the bottom portion thereof to reduce iron oxides, thereby obtaining reduced iron.
In recent years, of particular interest has been a process of manufacturing reduced iron in which a carbonaceous material, such as coal, is used as a reductant in place of natural gas. Such a method has already been put into practice and is referred to as an SL/RN method in which indurated pellets manufactured from iron ore are subjected to reduction through the application of heat using coal as a reductant.
Another reducing iron-making process is disclosed in U.S. Pat. No. 3,443,931, in which a mixture of pulverized iron ore and pulverized coal are agglomerated, and the agglomerated mass is subject to reduction through the application of heat on a rotary hearth, in high temperature atmosphere, yielding reduced iron.
Reduced iron obtained using the above-mentioned methods is charged into an electric furnace directly as source iron or in the form of briquettes. With the increasing trend of recycling scrap in recent years, this reduced iron is of particular interest, since it may be used as a diluent of impurities contained in the scrap.
A conventional method, however, does not involve separating slag components such as SiO2, Al2O3, and CaO contained in the iron ore or the like and in the carbonaceous material (coal or the like), from the molten iron produced. Therefore, the resultant reduced iron has a relatively low iron content (iron purity of metallic iron). In actual practice, these slag components are separated and removed during a subsequent refining process. However, an increase in the amount of slag not only decreases yield of refined molten iron, but significantly increases the running cost of an electric furnace. Therefore, reduced iron is required to be iron rich and have a relatively low content of slag components. In order to meet this requirement, it is necessary for the above-mentioned conventional reducing iron-making methods to use iron-rich iron ore, which narrows the choice of source materials for making iron.
Furthermore, a goal of the conventional methods described above is to obtain a reduced solid product as an intermediate product in an iron making process. Therefore, additional steps such as conveyance, storage, forming briquettes, and cooling are required before reduced iron is sent to the next refining process. These steps involve a large energy loss, and a briquetting step requires excess energy and a special apparatus.
In addition, a smelting reduction process such as the DIOS method is known in which iron oxides are directly reduced to obtain molten iron. In this method, iron oxides are pre-reduced to an iron purity of approximately 30 to 50%, and then molten iron in an iron bath is subjected to a direct reducing reaction with carbon, to obtain metallic iron. However, this method has problems; since two steps are required, pre-reduction and final reduction within an iron bath, the work is complicated, and in addition, due to direct contact between molten iron oxide (FeO) present in an iron bath and the refractory of a furnace, the refractory is significantly damaged.
Japanese Patent Publication (kokoku) No. 56-19366 discloses a method in which an agglomerate of metal oxide, a solid carbonaceous material, and slag materials is reduced through the application of heat to thereby enclose reduced metal with slag shell while maintaining the shape of the agglomerate, and then the slag shell is melted to separate metal from slag. This method must generate a sufficient amount of slag to completely enclose reduced metallic iron in order to prevent the metallic iron from being re-oxidized. Thus, the slag materials content must be increased. Furthermore, this method is likely to generate slag having a relatively high FeO content, which raises a serious problem, in practical application, of significantly damaging the refractory lining of equipment.
Thus, it is quite important to realize a method of making metallic iron having a relatively low content of slag components, since such a method adds more value to a metallic iron product, reduces the running cost of an electric furnace, and provides a flexible choice of source materials.
Since slag having a relatively large iron oxide content melts refractory, it is very important for industrial feasibility of this kind of iron-making process to reduce the iron oxide content of slag, generated accompanyingly in a process of reduction, in order to minimize damage to refractory.
The present invention has been achieved in view of the foregoing. An object of the present invention is to provide a method and apparatus of making metallic iron in which metallic iron, in either solid or molten form, having a very high purity, is readily and efficiently made from iron ore having a relatively low iron content or having a relatively high iron content, without damaging the refractory of a furnace via direct contact with molten iron oxide.
In the method of making metallic iron according to the present invention, iron oxide compacted with a carbonaceous reductant is subjected to reduction through the application of heat to yield metallic iron, the method having the following aspects:
(1) A shell containing metallic iron is generated and grown via reduction through the application of heat. The reduction normally is continued until substantially no iron oxide is present within the shell, during which slag aggregates within the shell.
(2) A metallic iron shell is generated and grown via reduction through the application of heat, the reduction is continued until substantially no iron oxide is present within the shell, and heating is further continued to allow slag generated within the shell to flow out from inside the shell.
(3) A metallic iron shell is generated and grown via reduction through the application of heat, the reduction is continued until substantially no iron oxide is present within the shell, and heating is further continued to allow molten metallic iron to separate from molten slag.
(4) A metallic iron shell is generated and grown via reduction through the application of heat, and the reduction is continued until substantially no iron oxide is present within the shell, during which slag aggregates within the shell, and then the aggregated slag is separated from metallic iron.
In order to embody aspect (2) described above, part of the metallic iron shell may be melted to allow molten slag to flow out from inside the shell. In this case or in order to embody aspect (3) described above, carburization may be continued within the metallic iron shell in the presence of a carbonaceous reductant so as to reduce the melting point of the metallic iron shell, thereby readily melting part or the entirety of the metallic iron shell.
When any of aspects (1) to (4) described above is embodied, a maximum temperature of heating for reduction may be controlled to be not less than the melting point of the accompanying slag and not more than the melting point of the metallic iron shell, so as to more efficiently conduct the reaction of generating metallic iron. This reducing step may be solid phase reduction, through which an iron oxide is reduced, and liquid phase reduction which is continued until substantially no iron oxide, composed mainly of FeO, is present, whereby the purity of the metallic iron obtained can be efficiently improved.
As used herein, the term xe2x80x9creduction is continued until substantially no iron oxide is present within the metallic iron shellxe2x80x9d means, on a quantitative basis, that the reduction through the application of heat is continued until the content of iron oxide, composed mainly of FeO, is preferably reduced to 5% by weight or less, more preferably to 2% by weight or less. From a different point of view, this means that the reduction through the application of heat is continued until the content of iron oxide, composed mainly of FeO in the slag separated from metallic iron, is preferably not more than 5% by weight, more preferably 2% by weight or less.
The thus-obtained metallic iron having a high iron purity and accompanying slag may be melted by further heating so as to separate one from the other through differences in their specific gravities. Alternatively, they may be solidified by chilling, and then crushed to separate the metallic iron from the slag magnetically, or by any other screening method. Thus, it is possible to obtain metallic iron having a high iron purity, with a metallization ratio of not less than 95%, or in some cases of not less than 98%.
In carrying out the above-described method of the present invention, the compact of iron oxide containing a carbonaceous reluctant may be granular or agglomerate, and be reduced through the application of beat in a manner having any of the following aspects:
1) The compact is moved in a horizontal direction.
2) The compact is placed on an iron belt, comprising walls formed at both edge portions thereof to prevent the compact from falling off the iron belt, and is moved in a horizontal direction.
3) The compact is placed on a horizontal surface.
4) The compact is tumbled.
5) The compact falls downward.
In addition, the compact may be elongated and reduced through the application of heat in a manner having any of the following aspects:
6) The elongated compact is moved downward in a vertical position.
Aspect 6) may be embodied as follows:
6-1) The elongated compact is continuously prepared and fed into a section where reduction is performed through the application of heat, the elongated compact comprising:
6-1-1) a support mesh made of iron and wrapping the elongated compact, or
6-1-2) an iron bar serving as a core thereof.
The above iron mesh or bar is preferably employed because it prevents the elongated compact from breaking at an intermediate position thereof due to its own weight while the elongated compact is moving downward.
7) The elongated compact is moved downward along a sloped surface.
Aspect 7) may be embodied as follows:
7-1) The elongated compact is placed on an iron belt and continuously fed into a section where reduction is performed through the application of heat.
Through employment of any of the above aspects, the aforementioned method of making metallic iron is more efficiently carried out.
An apparatus for making metallic iron according to the present invention carries out the above-described method of making metallic iron and has the following basic structure.
An apparatus for making metallic iron by reducing a compact of iron oxide containing a carbonaceous reducing agent through the application of heat comprises:
a thermal reduction apparatus for reducing the compact through the application of heat, thereby forming a shell comprising metallic iron and slag inside the shell;
a melting apparatus for melting the shell and the slag; and
a separator for separating the molten iron from the molten slag.
In the above-described apparatus for making metallic iron, when the compact is granular or agglomerate, the above-described thermal reduction apparatus may comprise a mechanism for a reducing the compact through the application of heat while moving the compact in a horizontal direction. A preferred embodiment of the mechanism is an endless rotary member, comprising an endless rotary member and a hearth located on the member and used for placing the compact thereon. Separating members may be provided on the hearth at certain intervals to prevent the compact from adhering to another compact. The separating members are preferably formed of a desulfurizing agent, so that desulfurization can also be performed in a process of reduction through the application of heat.
The above-described mechanism may also be embodied in the form of an iron belt, comprising walls formed at both edge portions thereof to prevent the compact from falling off the iron belt, for conveying thereon the compact in a horizontal direction and for subjecting the compact to reduction through the application of heat during the horizontal conveyance of the compact.
A preferred embodiment of the above-described melting apparatus may comprise a sloped floor for tumbling or sliding the reduced compact thereon and for melting the tumbling or sliding compact through the application of heat.
When the compact is granular or agglomerate, another preferred embodiment of the thermal reduction apparatus may comprise a feeding member, comprising a horizontal plane, for intermittently feeding in the compact placed on the horizontal plane, a discharging member for discharging the compact from the feeding member, and a heating mechanism for heating the compact. The discharging member may be a tilting member for making the position of the feeding member alternate between a horizontal position and a sloped position, or a pushing member for pushing out the compact from the feeding member, thereby smoothly discharging the compact.
An iron support may be placed on the feeding member and adapted to be discharged together with the compact. Separating members (preferably formed of a desulfurizing agent) are preferably provided on the feeding member at certain intervals to prevent the compact from adhering to another compact.
A preferred embodiment of the feeding member may comprise an iron belt for continuously conveying the compact thereon and for subjecting the compact to reduction through the application of heat. This avoids a problem that part of the reduced compact melts and adhesively accumulates on the internal surface of a furnace. When this embodiment is employed, the iron belt used for feeding in the compact is melted with reduced metallic iron to become molten iron.
A preferred embodiment of the aforementioned melting apparatus may comprise a sloped floor for melting the compact by application of heat while tumbling or sliding the compact thereon.
For more efficient reduction through the application of heat, the aforementioned thermal reduction apparatus may preferably comprise:
a mechanism for reducing the compact through the application of heat while tumbling the compact, or
a mechanism of tumbling, comprising a tumbling surface for tumbling the compact thereon and a discharging unit for discharging the compact from the tumbling surface, and a thermal reduction member for heating the compact.
The above-described thermal reduction apparatus and the melting apparatus may be integrated into a thermal reduction-melting apparatus, which comprises a mechanism of tumbling, comprising a sloped tumbling surface for gradually tumbling down the compact along a sloped direction and a discharging section for discharging the compact from the sloped tumbling surface, and a mechanism for reducing and melting the compact through the application of heat. This enables reduction and melting through the application of heat to be performed continuously and efficiently.
In the above-described thermal reduction-melting apparatus, the tumbling surface preferably comprises the interior surface of a channel-like member having an arc-shape, V-shape, or the like recess and is sloped along the length of the channel-like member. This enables smoother reduction and melting through the application of heat.
A further embodiment of the thermal reduction apparatus which receives the granular or agglomerate compact may comprise a mechanism for allowing the compact to fall downward and for reducing the falling compact through the application of heat. Alternatively, the thermal reduction-melting apparatus integrally comprising the thermal reduction apparatus and the melting apparatus may further comprise a space for allowing the granular compact to fall downward and a heating member for reducing and melting the granular compact through the sequential application of heat while the granular compact is falling.
The separator preferably comprises a submerged weir for receiving molten slag and molten iron falling from above on one side thereof and for releasing the molten slag from one side thereof and the molten iron from the other side thereof. Thus, the molten iron and the molten slag are continuously and readily separated one from the other.
When an elongated compact is used, the thermal reduction apparatus may comprise a mechanism for reducing the elongated compact through the application of heat while moving the elongated compact downward in a vertical position or along a downward sloped surface. This allows the elongated compact to be continuously reduced through the application of heat while it moves downward in a vertical position or along the downward sloped surface.
When the elongated compact is used, the elongated compact may be continuously fed onto an iron belt through an feeder, so that the elongated compact on the iron belt is continuously conveyed into a thermal reduction apparatus, where the elongated compact is reduced through the application of heat. In this case, the iron belt is also melted in a melting process with metallic iron generated in the reducing process, and collected in the form of molten iron.
Preferably, the apparatus for making metallic iron according to the present invention may further comprises means for feeding an iron belt for conveying the compact thereon, thereby feeding the compact on the iron belt into the thermal reduction apparatus and a melting apparatus for reducing and melting the compact through the application of heat. In this case, when the compact is granular or agglomerate, the iron belt may comprise walls formed at both edge portions thereof to prevent the compact from falling off the iron belt and may convey the compact thereon in a horizontal direction within the thermal reduction apparatus for reducing the compact through the application of heat. When the compact is in an elongated form, there may be provided forming means for continuously forming the elongated compact and for feeding the elongated compact onto the iron belt, thereby continuously forming the elongated compact and subjecting it to reduction and melting through the application of heat. The iron belt used is melted in the melting apparatus to thereby be merged with metallic iron, generated through reduction, and collected in the form of molten iron.